Sheet materials used in making medical drapes, medical gowns and absorbent articles, such as diapers and sanitary napkins, must be comfortable, soft, pliable and substantially liquid impermeable. Manufacturing and use requirements for such products often demand that the sheet material also be strong and durable. The sheet materials used in medical apparel and absorbent articles function to contain the discharged materials and/or to isolate these materials from the body of the wearer or from the wearer's garments and bed clothing. An ideal sheet material for use in medical apparel and absorbent articles will exhibit a high moisture vapor transmission rate that will reduce the build up of heat and humidity inside garments and articles made from the material. The ideal sheet material will also exhibit excellent barrier properties so as to prevent the passage or seepage of fluids, and will even prevent the passage of bacteria and viruses. Finally, the ideal material must be strong enough so that it does not rip or delaminate under normal usage conditions regardless of whether the material is dry or wet.
It is known that the exterior of the sheet material used in medical apparel and absorbent articles can comprise a flexible, fluid and vapor impervious cover to prevent fluids from passing through garments and articles made from the sheet material. These outer covers are often constructed from fluid and vapor impervious films such as polyethylene. While these films do an admirable job of containing liquids, they are not pleasing to the touch and they do not readily pass moisture vapor, which makes garments and articles made with such films uncomfortable and irritating to the wearer.
Certain polymeric films have been made more acceptable for apparel and personal care applications by creating micropores in the films to make breathable (i.e., moisture vapor permeable) microporous films. In microporous films, moisture is transported through the films by way of small gaps or holes in the film. One notable microporous film composite is made from polytetrafluoroethylene that is adhered to a textile material with an adhesive, as disclosed in British Patent Application No. 2,024,100. Microporous films adhesively bonded to textile substrates have been used in a variety of apparel products, including absorbent articles, as disclosed in PCT Patent Publication Nos. WO 95/16562 and WO 96/39031.
Laminates of a microporous film and a fibrous textile substrate have a number of disadvantages, including that such laminates permit some seepage of fluids when used as the backsheet in an absorbent article. For example, when microporous film laminates are used as the backsheet of a disposable diaper, the backsheet may permit the transmission of some urine through the pores in the backsheet when an infant wearing the diaper sits down. Liquid seepage through microporous film laminates is especially likely to occur when the microporous laminate is exposed to a fluid with a low surface tension, as for example when urine in a diaper is exposed to surfactants within the diaper itself. In addition, liquid seepage worsens as moisture vapor transmission increases. This is a result of an increase in pore size or in the actual number of pores.
When fluids seep through the pores of a microporous film, bacteria, viruses, and other microbes can pass through the film along with the fluids. Likewise, the passage of fluids through laminates made with microporous films, whether the fluids are liquid or gaseous, also increases the odors that emanate from such laminates. Microbial adsorbents have been added to some microporous films in an attempt to capture microbes passing through such films, as disclosed in PCT Patent Publication No. WO 96/39031. However, it is difficult to distribute microbial adsorbents throughout a microporous film in a manner that will adsorb all microbes seeping through the holes in the film. Likewise, microbial adsorbents are unlikely to prevent the passage of odors through the pores in a microporous film.
Moisture vapor permeable films comprised of polyether block copolymers, like the film disclosed in U.S. Pat. No. 4,493,870, have an advantage in apparel and personal care applications because such films are non-porous and therefore substantially impermeable to fluids, but they permit the passage of moisture vapor. U.S. Pat. Nos. 4,725,481; 5,422,172; and 5,445,874 disclose that moisture vapor permeable polyether block copolymer films can be attached to a variety of fibrous substrates including polyester, polypropylene and nylon. Bonding methods used to join the polyether block copolymer films to the fibrous substrates include adhesive lamination, thermal lamination and extrusion coating. Adhesive lamination and thermal lamination are generally carried out in a two step process whereby the film is first formed and is subsequently laminated to the fibrous substrate. With extrusion coating, a melted film is extruded directly onto a fibrous substrate and then passed through a nip while the film is still hot in order to press the film into engagement with the fiber network of the fibrous sheet.
Adhesive lamination, thermal lamination and extrusion coating methods have all been used to produce composite sheets of a fibrous nonwoven substrate and a moisture vapor permeable, substantially liquid impermeable film. It has been possible to make such composite sheets with good barrier properties so long as the moisture vapor permeable film is relatively thick (i.e., &gt;25 microns). However, it has not been possible to make such composite sheets with thinner films without sacrificing important barrier properties. Very thin moisture vapor permeable films are desirable in a composite sheet because thinner films facilitate significantly greater flux of moisture vapor through the composite sheet and because thinner films use less of the film material and are accordingly less expensive to produce.
Adhesive lamination is carried out in a post film formation step. For adhesive lamination to be feasible, the moisture vapor permeable film must have enough structure, tensile strength and tear strength such that the film can be formed, wound onto a roll, and later unwound and handled during the adhesive lamination process. It is extremely difficult to handle moisture vapor permeable films less than 25 microns (1 mil) in thickness during the adhesive lamination process without introducing holes into the film. Thus, when adhesive lamination has been used to attempt to make composite sheets with thinner films, the composite sheets have not exhibited the fluid barrier properties (e.g., hydrostatic head, dynamic fluid transmission) desirable for a composite sheet designed for use in absorbent articles or medical apparel.
Thermal lamination of moisture vapor permeable films less than 25 microns thick has similarly resulted in composite sheet materials with inadequate barrier properties. When composite sheets are made by thermally laminating a thin film to a fibrous substrate, the thin film handling problems associated with adhesive lamination as described above are encountered. In addition, to carry out a thermal lamination, the film must be subjected to elevated temperatures and pressures so as to soften the film and force it into mechanical engagement with the fibrous substrate. Generally, the peel strength between the film and the fibrous substrate increases with increasing extrusion melt temperatures and increasing nip pressures. Unfortunately, when moisture vapor permeable films with a thickness of less than 25 microns are subjected to the elevated temperatures and pressures needed to obtain adequate peel strength in the composite sheet, small holes develop in the film such that the composite sheet does not exhibit the fluid barrier properties desired in a composite sheet for use in absorbent articles or medical apparel. These small holes can result from the non-uniform temperature throughout the web during bonding and from high bonding pressures used in the prior art.
Extrusion coating processes disclosed in the prior art are similarly unable to generate a composite sheet with a thin moisture vapor permeable film of less than 25 microns that also has the barrier properties and moisture vapor transmission properties desirable for use in medical apparel and absorbent article applications. In an extrusion coating process, the polymer that forms the film is melted at an elevated temperature to reduce its viscosity such that when the polymer melt is coated onto the fibrous substrate and passed through a nip, the melt is pressed into engagement with the fibrous network of the substrate. Unfortunately, the low viscosity of the melted polymer, the pressure of the nip, and a thinness of the film each contribute to the generation of small holes in the film. In addition, thinner films are more susceptible to fiber protrusion through the film which also contributes to small holes.
Accordingly, there is a need for a composite sheet material that acts as a barrier to fluids, yet is also highly permeable to moisture vapor. There is also a need for a sheet material that readily transmits moisture vapor, but significantly deters the passage of bacteria, viruses and odors associated with such fluids. There is a further need for such a moisture vapor permeable, fluid impermeable composite sheet material that is also durable, strong, and flexible enough to be used in apparel and absorbent articles, and can be produced in an economical fashion, i.e., film extrusion and lamination in one process. Specifically, there is a need for a composite sheet material with a moisture vapor permeable film that is less than 25 microns thick, exhibits excellent moisture vapor transmission, high peel strength, and barrier properties sufficient to prevent passage of liquids under static and dynamic loading conditions. Finally, there is a need for a process for producing such a composite sheet material.